This application generally relates to composite articles that are joined together using brazes. More particularly, the invention described herein relates to bonded niobium-based silicide and molybdenum-based silicide composite articles that are joined together using silicon- and germanium-based brazes.
Nickel (Ni)-based superalloys have been used as jet engine materials for many years. The surface temperatures at the hottest locations of state-of-the-art jet engine turbine airfoils now approach 1,150xc2x0 C., which is approximately 85% of the melting temperatures of Ni-based superalloys. Niobium (Nb) and molybdenum (Mo) based refractory metal intermetallic composites (hereinafter referred to as xe2x80x9cNb-based RMICsxe2x80x9d and xe2x80x9cMo-based RMICs,xe2x80x9d) have much higher potential application temperatures, provided that they can be used at approximately 80% or more of their melting temperatures, which are generally greater than about 1700xc2x0 C.
Complex silicide-based RMICs that are made from Nbxe2x80x94Sixe2x80x94Tixe2x80x94Hfxe2x80x94Crxe2x80x94Al alloys or Moxe2x80x94Sixe2x80x94Bxe2x80x94Cr alloys appear to have the potential to become the next generation turbine materials with a long term, high-temperature capability that is significantly higher than that of current Ni-based superalloys. Because of their high melting temperatures, however, direct casting of hollow engine components with cooling channels from these Nb- and Mo-based RMICs is expected to be very difficult. At such high temperatures, very few materials can serve as casting cores and molds without experiencing creep, cracking, or reactions with the molten metals and thus contaminating the melt and degrading the cores. One potential alternative technique for the manufacture of complex-shaped components (e.g. airfoils) with cooling channels is to bond together, typically using brazes, two or more structural members that have been machined to the appropriate shapes. Currently, however, no such braze materials exist for these Nb- and Mo-based RMICs.
It is known in the art to make hollow components, such as turbine blades, by joining and bonding halves or multiple pieces together. However, the prior-art braze materials that have been developed for Ni-based or Fe-based alloys are not suitable for use with the new Nb- and Mo-based RMICs, which have very different alloy compositions and much higher working temperatures. Detrimental interactions are known to occur between nickel brazes, for example, and Nb-based RMICs.
Accordingly, there is a need in the art for improved high temperature composite articles that are joined together using brazes.
The present invention meets this and other needs by providing articles formed from Nb- and Mo-based RMICs that are joined together by either a silicon- or germanium-based braze. Silicon- and germanium-based brazes for joining Nb- and Mo-based RMICs are also disclosed.
Accordingly, one aspect of the invention is to provide an article having a melting temperature of at least about 1500xc2x0 C. The article comprises a first piece and a second piece joined by a braze to the first piece. The first piece comprises one of a first Nb-based RMIC and a first Mo-based RMIC, wherein the first Nb-based RMIC comprises titanium, hafnium, silicon, chromium, and niobium, and the first Mo-based RMIC comprises molybdenum, silicon, and at least one of chromium and boron. The second piece comprises one of a second Nb-based RMIC and a second Mo-based RMIC, wherein the second Nb-based RMIC comprises titanium, hafnium, silicon, chromium, and niobium, and the second Mo-based RMIC comprises molybdenum, silicon, and at least one of chromium and boron.
A second aspect of the invention is to provide an airfoil having a melting temperature of at least about 1500xc2x0 C. The airfoil comprises a first piece and a second piece joined by a braze to the first piece. The first piece comprises one of a first Nb-based RMIC and a first Mo-based RMIC, wherein the Nb-based RMIC comprises titanium, hafnium, silicon, chromium, and niobium, and the first Mo-based RMIC comprises molybdenum, silicon, and at least one of chromium and boron. The second piece comprises one of a second Nb-based RMIC and a second Mo-based RMIC, wherein said second Nb-based RMIC comprises titanium, hafnium, silicon, chromium, and niobium, and the second Mo-based RMIC comprises molybdenum, silicon, and at least one of chromium and boron.
A third aspect of the invention is to provide an airfoil having a melting temperature of at least about 1500xc2x0 C. and comprising a first piece and a second piece joined by a braze to the first piece. The first piece comprises one of a first Nb-based RMIC and a first Mo-based RMIC, wherein the Nb-based RMIC comprises titanium, hafnium, silicon, chromium, and niobium, and the first Mo-based RMIC comprises molybdenum, silicon, and at least one of chromium and boron. The second piece comprises one of a second Nb-based RMIC and a second Mo-based RMIC, wherein the second Nb-based RMIC comprises titanium, hafnium, silicon, chromium, and niobium, and the second Mo-based RMIC comprises molybdenum, silicon, and at least one of chromium and boron. The braze joining the first piece to the second piece comprises one of germanium and silicon, and one of chromium, titanium, gold, aluminum, palladium, platinum, and nickel.
A fourth aspect of the invention is to provide a turbine assembly having at least one component having a melting temperature of at least about 1500xc2x0 C. and comprising a first piece and a second piece joined by a braze to the first piece. The first piece comprises one of a first Nb-based RMIC and a first Mo-based RMIC, wherein the Nb-based RMIC comprises titanium, hafnium, silicon, chromium, and niobium, and the first Mo-based RMIC comprises molybdenum, silicon, and at least one of chromium and boron. The second piece comprises one of a second Nb-based RMIC and a second Mo-based RMIC, wherein the second Nb-based RMIC comprises titanium, hafnium, silicon, chromium, and niobium, and the second Mo-based RMIC comprises molybdenum, silicon, and at least one of chromium and boron. The braze joining the first piece to the second piece comprises one of germanium and silicon, and one of chromium, titanium, gold, aluminum, palladium, platinum, and nickel.
Finally, a fifth aspect of the invention is to provide a method of making an article having a melting temperature of at least about 1500xc2x0 C. and comprising a first piece and a second piece that are joined together by a braze. The first piece and second piece each comprise one of a Nb-based RMIC and a Mo-based RMIC, wherein the Nb-based RMIC comprises titanium, hafnium, silicon, chromium, and niobium and the Mo-based RMIC comprises molybdenum, silicon, and at least one of chromium and boron. The method comprises the steps of: providing the first piece and the second piece such that the first piece and the second piece form a an interface therebetween; providing a braze to the interface between the first piece and the second piece, the braze having a melting temperature and comprising one of germanium and silicon and one of chromium, titanium, gold, aluminum, palladium, platinum, and nickel; heating the first piece, the second piece, and the braze to a first temperature for a first predetermined hold time, the first temperature being at least 20xc2x0 C. above the melting temperature of the braze; and further heating the first piece, the second piece, and the braze to a temperature between about 1300xc2x0 C. and about 1450xc2x0 C. for a second predetermined hold time, thereby joining the first piece and the second piece at the interface and forming the article.
These and other aspects, advantages, and salient features of the present invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims.